The present invention concerns in particular low-cost infrared (IR) gas detection. A standard technology in this field consists in a thermal IR light source, an interference line filter, a sample chamber and an IR detector. The line filter corresponds to the characteristic absorption wavelength of the gas to be detected so that only light of this specific wavelength is incident onto the detector. If a gas to be detected is present in the sample chamber, part of the light is absorbed by the gas and the detector signal is lowered subsequently. In order to take into account the intensity variation of the light source due to aging, moisture or dirt, a part of the emitted light is directed outside the sample chamber onto a reference detector (so-called two-beam or reference-beam technique).
Such non-diffractive IR (NDIR) gas detectors suffer from two drawbacks. Firstly, thermal light sources have a high power consumption and a low light efficiency which makes battery-driven operation difficult and implies cooling issues. Secondly, the center wavelength of interference line filters is dependent on the temperature so that for different ambient temperatures, the detection operates at different positions of the gas absorption peak which in turn makes calibration difficult.
Recent developments with Vertical Cavity Surface Emitting Lasers (VCSEL) have shown a way to improve low-cost single gas detectors. VCSEL wavelengths are precisely defined and can be tuned over a few nanometers by a change of the VCSEL drive current. Such VCSEL diodes are meanwhile available for the near infrared (NIR) wavelength range of 1.3–2.05 μm. Many of the gases detected by IR absorption have the first or second overtones of their absorption peaks in this wavelength range. Although these overtones are substantially weaker than the fundamental peaks, gas detection is very sensitive as VCSELs typically supply about 1000 times more light intensity than a thermal light source. An important advantage of VCSELs is their low power consumption of a few Milliwatt compared to a few Watt for thermal light sources.
A main difference between a standard NDIR detection and detection based on VCSELs is that NDIR techniques have a low spectral resolution and therefore measure gas absorption peaks which are typically several 100 nm wide. These broad absorption peaks are in fact composed of a large number of very sharp absorption lines. VCSELs emit with a very sharp wavelength peak which can be modulated within a few nanometers. For this reason, a VCSEL-based gas detector measures one single absorption line instead of a broad absorption peak.
Several authors have described a gas detection set up with a VCSEL source where the wavelength of the VCSEL is scanned across the absorption line of the gas as represented in FIG. 2. This scanning is done with a given modulation frequency F. This modulation is achieved by imposing a small alternating current (100 μA typically) of frequency F onto a constant current above the lasing threshold (some mA typically). For some measurement techniques, this “constant current” is slowly swept across the whole operation range of the VCSEL in order to detect subsequent absorption lines. With such a set up, a line filter is no longer needed which is an important cost reduction factor for low-cost products.
The present invention is based on a source formed by a wavelength modulated VCSEL and uses the fact that the modulation of the wavelength is directly connected to a modulation of the VCSEL output intensity. The intensity of the light having passed the gas volume and being incident on the detector therefore shows a first modulation related to the VCSEL intensity and a second modulation related to the gas absorption as the wavelength is scanned across the gas absorption line.
With a standard IR detector which delivers a signal proportional to the incident radiation, the signal treatment consists in measuring the detector signal by a lock-in technique on twice the modulation frequency (2F-detection). By this, the DC signal component—which stems from the offset light detected throughout the modulation range—is suppressed. However, a reference beam has still to be used in order to obtain information about the overall light intensity of the initial light beam provided by the source for obtaining a precise value of the gas concentration. This reference beam is usually detected by a second specific detector. Thus, the generation and the detection of a reference beam complicate the device and increase its production cost.
U.S. Pat. No. 6,356,350 B1 describes a method and an apparatus for demodulating a plurality of frequency components output from a photodetector in a wavelength modulation spectroscopy system and determining absorption line shapes from the demodulated data. The method allows information about the absorber line shape and line width, gas concentration measurement over a range of gas pressures temperatures and concentrations. For this, at least two even harmonics or a plurality of an harmonics of the wavelength modulation frequency F are necessary. In general, the prior art document teaches to use more even harmonic demodulated frequency components than other frequency components. The method disclosed in U.S. Pat. No. 6,356,350 B1 is not appropriate for providing a gas detector device with low fabrication costs for large series which allows an efficient gas concentration measuring or presence of a gas.
An object of the present invention is to provide an efficient gas concentration measuring device or detector at low cost. In particular, the aim of the present invention is to solve the above mentioned problem relative to the reference beam.